In recent years, systems have been proposed to increase the safety of a vehicle by providing a driver the ability to monitor a periphery of a vehicle by capturing images outside a vehicle periphery and displaying the captured images. For example, a vehicle-periphery-monitoring apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2003-169323 obtains at least two bird's-eye-view images by determining a viewpoint above the vehicle by using at least two cameras to capture images outside the vehicle periphery, combining the bird's-eye-view images into a single image, adding a mask pattern to a portion where the bird's-eye-view images are joined to each other, as a border image, and displaying the single image. FIG. 11 shows an example image displayed by a known vehicle-periphery-monitoring system. As shown in FIG. 11, the displayed image may include a bird's-eye-view image 201 of a vehicle illustrated in the center of the displayed image, a bird's-eye-view image 212 showing an area ahead of the vehicle, a bird's-eye-view image 214 showing an area to the right of the vehicle, a bird's-eye-view image 216 showing an area in the rear of the vehicle, and a bird's-eye-view image 218 showing an area to the left of the vehicle, all of which are combined with one another. Further, a mask pattern 252 is added to a portion where the bird's-eye-view images 212 and 214 are joined to each other, a mask pattern 254 is added to a portion where the bird's-eye-view images 214 and 216 are joined to each other, a mask pattern 256 is added to a portion where the bird's-eye-view images 216 and 218 are joined to each other, and a mask pattern 258 is added to a portion where the bird's-eye-view images 218 and 212 are joined to each other.
Further, the vehicle-periphery-monitoring apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2003-169323 is provided with an ultrasonic sensor configured to detect an object (obstacle) existing in blind spots of the displayed image such as the real space corresponding to the portion where the bird's-eye-view images are joined to each other. When the ultrasonic sensor detects the obstacle, the mask pattern added to the corresponding joining portion flashes. For example, in FIG. 11, if an object exists in the real space corresponding to the portion where the bird's-eye-view image 216 showing the rear area and the bird's-eye-view image 218 showing the left area are joined to each other, the mask pattern 256 flashes. The driver can confirm, for example, an obstacle existing in the periphery of the vehicle by viewing the image shown in FIG. 11.
However, even though the vehicle-periphery-monitoring apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2003-169323 allows the driver to identify the existence of an object through the flashing mask pattern added to the joining portion, it is difficult for the driver to identify the shape and the specific position of the object. Further, if the posture of the vehicle is changed, namely, if the vehicle is inclined toward the front due to a person in the front seat of the vehicle, the range of the camera affixed to the vehicle is changed. Further, if end portions of real spaces 232 and 234 corresponding to two adjacent bird's-eye images are separated from each other so that a blind spot occurs and an object 202 exists in the blind spot, as shown in FIG. 12, it is difficult for the driver to identify the object 202. Therefore, the driver is not necessarily capable of adequately monitoring the vehicle periphery.